To gain a molecular understanding of phagocyte activation processes, we previously cloned cDNAs and genes for the following G protein-coupled chemotactic receptors: the N-formylpeptide receptor, interleukin-8 receptors A and B (alpha chemokine receptors selectively expressed in neutrophils), and the MIP-1alpha/RANTES receptor (a beta chemokine receptor, also known as CCR1). We also previously established that open reading frame ECRF3 of Herpesvirus saimiri encodes an alpha chemokine receptor and open reading frame US28 of human cytomegalovirus encodes a beta chemokine receptor most specific for RANTES, suggesting a form of molecular mimicry by these viruses. We have now extended our studies of the chemokine receptors and have obtained the following main results: 1) IL-8 receptor A binds specifically to IL-8 whereas IL-8 receptor B is equiselective for IL-8 and at least four other alpha chemokines; 2) the two main regions in the ectodomains of IL8RB that diverge in sequence from IL8RB are both dominant determinants of the B specificity phenotype in chimeric A/B receptors; 3) low affinity chemokine binding supports efficient activation of IL-8 receptor chimeras; 4) the binding sites for the IL8RB ligands appear to overlap but are distinct, suggesting that both broad- and narrow-spectrum receptor subtype antagonists could be developed; 5) human and mouse CCR3 are eosinophil-specific and eotaxin-selective G protein-coupled receptors; 6) human CCR1 and CCR2B are both monocyte receptors for monocyte chemoattractant protein-3 (MCP-3), yet they have completely different selectivities for other beta chemokines; 7) we have cloned a cDNA for a monocyte and T lymphocyte beta chemokine receptor named CCR5 that, like CCR1 and CCR4, is also selective for MIP-1alpha and RANTES, but is the first receptor identified with high specificity for MIP-1beta; 8) CCR5 has been coopted by macrophage-tropic strains of human immunodeficiency virus-1 as a coreceptor for cell entry acting at the level of CD4-envelope dependent fusion (collaboration with Berger lab, LVD/NIAID). The fusion reaction could be inhibited in PBMCs, primary macrophages and cells transfected with CCR5 and CD4 by the same chemokines that bind selectively to CCR5 but no others; 9) C3a and C5a are potent and effective chemoattractants for a human mast cell line, acting via a pertussis toxin-sensitive signal transduction pathway. The sheer number of chemokines and chemokine receptors and the complex selectivities they have for each other and for leukocyte subsets attests to their fundamental importance in inflammation. The discovery of differential expression of chemokine receptors in leukocytes suggests they may be responsible for selective phagocyte accumulation in pathological states, making them good targets for development of cell type-selective anti-inflammatory drugs. The discovery of viral chemokine receptors and HIV coreceptors may provide new insights into viral pathogenesis.